CG settings, or best practices

I've got a small 250 sized copter, and can't get the CG centered.  It's about 10mm to the rear. 

It flies fine, and if I take off quickly, everything works properly.   If I try to take off slowly, it flips over to the heavy side. 

Are there any arducopter settings to tell it the CG is off? 
Otherwise, what's the best practice in this situation?  I can't move things due to the small frame size. 
So, I suppose adding weight is the only option, but I think an off CG is probably better than more weight. 

Thanks for any advice.

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  • 10mm should not be enough to cause your issues if you properly calibrated everything. That's way under a 1/2 inch and I've seen planes and whatnot WAY more than that in regards to the flight controller. I always use a string to hang my multirotors to determine just where there COG is and even on 250 frames with cameras and vTX's and everything am able to get the COG bang on. Takes a bit of fiddling for sure but it is worth the extra effort.

    • Hi, I've got a new related question... 

      Very small differences in CG definitely make a difference on this quad.  
      I'd like to know if this is normal, and if not, how to correct it.    I know ideally the CG should be perfect.  But this is the real world, and you mentioned 10mm should not be enough to cause my issues. 

      So, I moved the buzzer to one of the arms, as it was interfering w/ my compass.  It's a small, maybe 1cm round plastic buzzer.  Doesn't weigh much at all.  BUT, now my copter tries to flip over to that side if I take off slowly.  (Holding the copter between it's front and back 'points' (there are handy narrow points to hold the frame at the front and back) I can barely detect one side is heavier)

      I'm curious why the copter can fly and hover perfectly, no matter how unbalanced things are (w/in reason).  But, taking off it always tries to flip. I can slowly raise the throttle, and it slowly lifts one side of the copter, It seems like it should have plenty of data and time to figure out it's starting to flip over while taking off, and add thrust to the other motors, but it doesn't. 
      but, as I mentioned, if I just immediately throttle up to a little over 50%, it hops evenly off the ground and remains level. 
      Could I have some bad settings that are causing this behavior?

      • When you take off slowly you are fighting the ebb and flows of the propwash and the smaller the frame the more it has to fight the turbulence, When you punch it, it breaks the "seal" and is able to take off easier.

        You might try to move the speaker to the opposite arm on the opposite side and see if the issue follows the speaker and if so try a different speaker :)

    • Well I'm thinking size matters, so on a big plane, 10mm would be nothing, but on a small 250 size, 10mm starts to become noticeable. 
      It does fly fine once it's off the ground, but obviously the rear motors must be working harder. 

      I really can't move things anymore..  I'm using a big battery, which is the problem...  If I slide it ALL the way forward, the CG is ok, but it's pushing against my camera, so the camera is no longer isolated (on rubber mounts), as it's pressing against a solid potentially vibrating surface.  Moving the battery back several MM so it clears the camera puts my CG off. 

      Right now I've simply got a wire acting as my video antenna, and that's not the best..   If there aren't any settings that could help the CG issue, maybe I'll add weight by adding an SMA connector and switch to a circular polarized antenna.. That's at the front of the vehicle, and would probably be enough weight to get it all centered. 

      light as possible was my design goal, but maybe it's time for compromises. 

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